There are a number of imaging applications that require, at some stage in processing, the application of a uniform pressure onto the surface of a substrate. Conventional offset lithographic printing equipment, for example, uses applied pressure for transferring an ink colorant from a printing plate onto the print substrate. Various types of office duplicating equipment, such as xerographic copiers, use pressure from paired rollers for image transfer onto paper or other suitable substrate. Laminators, such as that used with the “KODAK APPROVAL Digital Proofing System”™ available from the Eastman Kodak Company, located in Rochester, N.Y., also require application of controlled pressure, commonly combined with heat, for transfer of an image from an intermediate medium to a paper substrate or for application of a protective coating over the substrate surface. Micro-encapsulated media, such as CYCOLOR™ print media, commercially available from Cycolor, Inc. and described in U.S. Pat. No. 5,916,727 (Camillus et al.), also require the controlled application of pressure for processing the final image from encapsulated leuco dye elements that are coated into an imaging substrate, after these elements are activated by exposure to light.
With each of these applications, conventional drum rollers apply pressure uniformly onto a surface. In operation, opposing drum rollers are forced against each other to provide, at their interface, a linear nip for pressure application. With roller rotation, the substrate moves through the nip. In practice, large drum rollers have proved to be an acceptable solution for lithographic printing and for many xerographic and other image transfer and lamination uses. As just a few examples: U.S. Pat. No. 6,658,230 (Satoh) discloses an apparatus for transferring imaging toner using paired rollers; U.S. Pat. No. 6,463,981 (Kerr) discloses the use of pressure rollers in a lamination apparatus; U.S. Pat. No. 5,777,650 (Blank) discloses a pressure roller for transferring and fixing an ink-based image onto a substrate; and U.S. Pat. No. 5,208,609 (Chung et al.) discloses the use of a pressure-roller apparatus with the Cycolor media.
However, for some applications, particularly where it is necessary to apply a substantial amount of pressure uniformly onto a substrate surface, such as when processing the Cycolor print media, there are drawbacks to the conventional use of paired rollers. Thus, for example, the paired roller solution of U.S. Pat. No. 5,208,609 may work adequately with small-format Cycolor images, but be less than satisfactory if scaled to accommodate larger images, such as 8×10 inch images. Acknowledged among the drawbacks to the use of rollers are problems due to surface imperfections, insufficient overall pressure between rollers, sag, eccentricity about axes of rotation, and non-uniform force distribution across the nip.
As an alternative to applying pressure using paired rollers, commonly-assigned U.S. Pat. No. 6,390,694 (Allen et al.) discloses a pressure assembly that oscillates a point contact mechanism over the substrate surface in raster fashion. In a similar spirit, U.S. Pat. No. 5,550,627 (Dowler et al.), noting the above-cited problems with conventional roller use for microencapsulated media processing, discloses a combined exposure and pressure applicator head that oscillates over the substrate, where the pressure applicator portion drives a small number of contact points along the surface in a raster pattern, thereby providing a more uniform pressure. The solution of U.S. Pat. Nos. 6,390,694 and 5,550,627 may yield a suitable image output, provided that mechanical tolerances are highly accurate. However, the slow speed of the oscillating point-contact approach makes this approach commercially unattractive. The deployment of multiple contact points scanned simultaneously, as is used in U.S. Pat. No. 5,550,627, may help to reduce the overall processing time somewhat. Even this solution, however, does not boost throughput speed sufficiently for commercialization.
For Cycolor media processing and for other types of apparatus used for forming images, intense localized pressure must be uniformly applied over the surface of a substrate. However, prior art approaches have not yet provided a robust, low cost solution to the problem of applying, onto a sheet of substrate having a range of possible dimensions, sufficient pressure with the needed level of uniformity and with the processing speed needed for imaging applications. Thus, it can be seen that there is a need for an apparatus and method for uniformly applying pressure onto a substrate.